Objective lens and optical pickup device

ABSTRACT

In order that thickness reduction, weight reduction, and mass productivity improvement should be achieved in an objective lens even in the case where NA is high, an objective lens according to the present invention is a bi-convex single lens having at least one aspheric surface, and satisfies conditions: (1) 3.5&lt;D H-S /D H-H′ &lt;4.3; (2) 3.5&lt;D H′-T2 /D T1-H &lt;50; (3) 0.9&lt;d/f&lt;1.1 (NA≧0.85). Here, D H-S  is a distance on the optical axis from a front principal point H to a focal point S on an optical information recording medium side; D H-H′  is a distance on the optical axis from the front principal point H to a rear principal point H′; D H′-T2  is a distance on the optical axis from the rear principal point H′ to an intersecting point T 2  of the optical axis and an optical information recording medium side surface of the objective lens; D T1-H  is a distance on the optical axis from an intersecting point T 1  of the optical axis and a light source side surface of the objective lens to the front principal point H; d is a thickness on the optical axis; and f is a focal length.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens used for performingrecording, reproduction and deletion of information on an opticalinformation recording medium, and to an optical pickup device employingthis objective lens.

2. Description of the Background Art

In the conventional art, optical disk units capable of writinginformation onto an optical disk such as a CD medium and a DVD medium,reading information from an optical disk, and deleting informationrecorded on an optical disk are widely used. In such optical disk units,an optical pickup device that includes an objective lens is employed(see, for example, Japanese Laid-Open Patent Publication No.2003-91854).

In recent years, Blu-ray Disc (registered trademark) employing a bluelaser has been developed and has caused the necessity of an objectivelens having a remarkably high numerical aperture (NA=0.85). When such anobjective lens is to be realized, the thickness of the objective lensneed remarkably be increased in general. This causes an increase in theload to the actuator for driving the objective lens. Further, a problemalso arises that the high NA causes notable occurrence of off-axialaberration and coma aberration resulting from decentering. Further, insuch an objective lens having a remarkably high NA, the inclinationangle of the lens surface increases. This causes difficulty infabrication of a molding die for objective lens manufacturing and inmolding of an objective lens.

Thus, an object of the present invention is provide an objective lens inwhich even in case of a remarkably high NA, thickness reduction andweight reduction are achieved, while satisfactory mass productivity isobtained.

SUMMARY OF THE INVENTION

In order to perform at least one of recording, reproduction and deletionof information on an optical information recording medium, an objectivelens according to the present invention is for converging light onto arecording surface of the optical information recording medium. Theobjective lens is a bi-convex single lens having at least one asphericsurface, and satisfies the following conditions.3.5<D _(H-S) /D _(H-H′)<4.3  (1)3.5<D _(H′-T2) /D _(T1-H)<50  (2)0.9<d/f<1.1  (3)

(here, a numerical aperture NA on the optical information recordingmedium side of the objective lens satisfies NA≧0.85)

where,

D_(H-S) is a distance [mm] on the optical axis from a front principalpoint H of the objective lens to a focal point S on the opticalinformation recording medium side of the objective lens,

D_(H-H′) is a distance [mm] on the optical axis from the front principalpoint H of the objective lens to a rear principal point H′ of theobjective lens,

D_(H′-T2) is a distance [mm] on the optical axis from the rear principalpoint H′ of the objective lens to an intersecting point T2 of theoptical axis and the optical information recording medium side surfaceof the objective lens,

D_(T1-H) is a distance [mm] on the optical axis from an intersectingpoint T1 of the optical axis and a light source side surface of theobjective lens to the front principal point H of the objective lens,

d is a thickness [mm] on the optical axis of the objective lens, and

f is a focal length [mm] of the objective lens.

Alternatively, an objective lens according to the present invention is abi-convex single lens having at least one aspheric surface, and maysatisfy the following condition.0.8<R1/r1<0.85  (4)0.9<d/f<1.1  (3)

(here, a numerical aperture NA on the optical information recordingmedium side of the objective lens satisfies NA≧0.85)

where,

R1 is a paraxial curvature radius [mm] on a light source side of theobjective lens,

r1 is an effective radius [mm] on the light source side of the objectivelens,

d is a thickness [mm] on the optical axis of the objective lens, and

f is a focal length [mm] of the objective lens.

Further, an optical pickup device according to the present invention isused for performing at least one of recording, reproduction and deletionof information on an optical information recording medium, and includesa light source and a converging optical system which includes any one ofthe above-mentioned objective lenses for converging a light beam emittedfrom the light source onto a recording surface of the opticalinformation recording medium.

According to the present invention, even when the NA is equal to orhigher than 0.85, thickness reduction and weight reduction are achievedin an objective lens and its mass productivity is also improved.

These and other objects, features, aspects and effects of the presentinvention will become further clear from the following detaileddescription with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a schematic configuration of an opticalpickup device according to Embodiment 1 of the present invention;

FIG. 2 is an optical path diagram of an objective lens shown in FIG. 1;

FIG. 3 is a diagram showing a schematic configuration of a computersystem according to Embodiment 2 of the present invention;

FIG. 4 is a block diagram showing a schematic configuration of anoptical disk drive shown in FIG. 3;

FIG. 5 is a longitudinal aberration diagram of an objective lensaccording to Numerical Example 1;

FIG. 6 is a lateral aberration diagram of an objective lens according toNumerical Example 1;

FIG. 7 is a longitudinal aberration diagram of an objective lensaccording to Numerical Example 2;

FIG. 8 is a lateral aberration diagram of an objective lens according toNumerical Example 2;

FIG. 9 is a longitudinal aberration diagram of an objective lensaccording to Numerical Example 3;

FIG. 10 is a lateral aberration diagram of an objective lens accordingto Numerical Example 3;

FIG. 11 is a longitudinal aberration diagram of an objective lensaccording to Numerical Example 4; and

FIG. 12 is a lateral aberration diagram of an objective lens accordingto Numerical Example 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIG. 1 is a diagram showing a schematic configuration of an opticalpickup device according to Embodiment 1 of the present invention.

An optical pickup device according to Embodiment 1 includes a lightsource 1, a collimate lens 3, a prism 4, an objective lens 5, and anactuator 7.

The light source 1 is composed, for example, of a semiconductor laser,and emits a light beam 2 having a wavelength in a range of 390 nm to 420nm. The light beam 2 emitted from the light source 1 is converted intoan approximately parallel light beam by the collimate lens 3. The lightemitted from the collimate lens 3 is refracted by the prism 4 in adirection perpendicular to the optical axis of the light source 1, andthen converged onto the optical information recording medium 6 by theobjective lens 5.

The objective lens 5 is coupled to the actuator 7 such that its centeraxis approximately coincides with the optical axis of the lightrefracted by the prism 4. Further, the objective lens 5 is movable in adirection perpendicular to the optical axis of the incident light bymeans of the actuator 7. Thus, even when the wavelength of the laserlight varies and thus the light beam becomes divergent or convergent,positional deviation of a spot in the track direction on the opticalinformation recording medium 6 can be corrected, that is, trackingcontrol can be performed.

FIG. 2 is an optical path diagram of the objective lens 5 shown inFIG. 1. The definitions of the symbols in FIG. 2 are as follows.

S is a focal position on the optical information recording medium sidein the case where a parallel light beam enters from the light sourceside,

T1 is an intersecting point of the light source side surface of theobjective lens and the optical axis,

T2 is an intersecting point of the optical information recording mediumside surface of the objective lens and the optical axis,

H is a front principal point of the objective lens,

H′ is a rear principal point of the objective lens,

d is a thickness on the optical axis of the objective lens,

f is a focal length of the objective lens,

R1 is a paraxial curvature radius on the light source side of theobjective lens,

r1 is an effective radius on the light source side of the objectivelens,

r2 is an effective radius on the optical information recording mediumside of the objective lens, and

α is a maximum angle formed between an incident light and a normal linethat passes a point on the light source side surface of the objectivelens, a height from the optical axis to the point on the light sourcesurface side being equal to or less than the effective radius on thelight source side (maximum inclination angle).

The effective radius indicates the radius of the cross section of alight beam that satisfies the NA required for the optical pickup device(0.85 in the case of Blu-ray Disc (registered trademark)).

Hereinafter, conditions that are preferable to be satisfied by theobjective lens according to the present embodiment are described. In thefollowing description, a plurality of conditions are set forth. It ismost preferable that an objective lens is formed so as to satisfy asmany conditions as possible. However, an objective lens which satisfiesany one of the following conditions and achieves the effectcorresponding to the condition may be obtained.

It is preferable that an objective lens according to the presentembodiment satisfies the following conditions simultaneously.3.5<D _(H-S) /D _(H-H′)<4.3  (1)3.5<D _(H′-T2) /D _(T1-H)<50  (2)0.9<d/f<1.1  (3)

(here, a numerical aperture NA on the optical information recordingmedium side of the objective lens satisfies NA≧0.85)

where,

D_(H-S) is a distance [mm] on the optical axis from the front principalpoint H of the objective lens to the focal point S on the opticalinformation recording medium side of the objective lens,

D_(H-H′) is a distance [mm] on the optical axis from the front principalpoint H of the objective lens to the rear principal point H′ of theobjective lens,

D_(H′-T2) is a distance [mm] on the optical axis from the rear principalpoint H′ of the objective lens to the intersecting point T2 of theoptical axis and the optical information recording medium side surfaceof the objective lens, and

D_(T1-H) is a distance [mm] on the optical axis from the intersectingpoint T1 of the optical axis and the light source side surface of theobjective lens to the front principal point H of the objective lens.

When the conditions (1) to (3) are satisfied, even in the case where anobjective lens has a high NA, the objective lens can be made thin and arequired working distance is ensured. Further, the occurrence ofoff-axial aberration and coma aberration resulting from decentering andthe like can be suppressed. In contrast, when each value falls outsidethe ranges of conditions (1) to (3), it is difficult to simultaneouslyachieve reduction in thickness and weight of the objective lens and thesuppression of aberration.

Further, it is preferable that an objective lens according to thepresent embodiment satisfies the following conditions simultaneously, inplace of the above-mentioned conditions (1) to (3), or in addition tothe above-mentioned conditions (1) to (3).0.8<R1/r1<0.85  (4)0.9<d/f<1.1  (3)

(here, a numerical aperture NA on the optical information recordingmedium side of the objective lens satisfies NA≧0.85)

When the conditions (3) and (4) are satisfied, even in the case wherethe objective lens has a high NA, thickness and weight of the objectivelens can be reduced and degradation in the converging characteristics,caused by the occurrence of off-axial aberration and coma aberrationresulting from decentering, can be suppressed. When R1/r1 is smallerthan 0.8, the maximum inclination angle α becomes excessively large,whereby causing notable difficulty in fabrication of a molding die andin molding of the lens. Further, when R1/r1 is greater than 0.85, theoccurrence of coma aberration resulting from decentering is increased.

Further, it is preferable that the objective lens according to thepresent embodiment satisfies the following condition.0.75<r2/r1<0.8  (5)

When r2/r1 is set to be greater than 0.75, concentration of light energyon the medium side surface of the objective lens can be reduced. Thus,even when light having high energy enters the objective lens, rise intemperature can be minimized, thereby reducing degradation in theoptical characteristics caused by the rise in temperature.

As material for forming the objective lens, plastic resin or glass isused. The above-mentioned merit in that the concentration of lightenergy can be reduced is advantageous especially in the case whereplastic is employed. Specifically, when light having a wavelength ofapproximately 400 nm is used, for example, in the case of Blu-ray Disc(registered trademark), the plastic material may be decomposed owing tothe concentration of light energy onto the objective lens. However, whenr2/r1 is set to be greater than 0.75 in accordance with the condition(5), the concentration of light energy can be reduced so thatreliability and durability of the objective lens can be improved.

When the light energy is highly concentrated, the rise in temperature ofthe objective lens becomes greater regardless of the material of theobjective lens, thereby leading to a problem of deviation of the focalposition. However, according to the condition (5), the concentration oflight energy can be reduced, whereby stability in the light convergingperformance can be ensured.

Here, when r2/r1 is equal to or smaller than 0.75, the effect ofreducing the concentration of light energy cannot be satisfactorilyobtained. In contrast, when r2/r1 is equal to or greater than 0.8, theoccurrence of coma aberration with respect to the decentering at thetime of molding is increased.

Further, it is preferred that the objective lens according to thepresent embodiment satisfies the following condition.60°<α<65°  (6)

When the condition (6) is satisfied, a practical design of the objectivelens, fabrication of a molding die, and molding using the molding diecan be actually performed, thereby improving the mass productivity. Incontrast, when the maximum inclination angle α falls outside the rangeof the condition (6), difficulty arises in the fabrication of a moldingdie and in the molding.

Further, it is preferred that the objective lens according to thepresent embodiment satisfies the following condition.WD/(d+2×r1)≧0.12  (7)

where,

WD is a working distance of the objective lens (that is, a distance fromthe optical information recording medium side surface of the objectivelens to the surface of the optical information recording medium).

As shown in FIG. 1, in order that the height (horizontal dimension inFIG. 1) of the optical pickup device should be reduced, the optical pathis bent at a right angle by using the prism 4. In this case, the heightof the optical pickup device is restricted by the sum of the thicknessof the objective lens 5 and the height of the prism surface necessaryfor refracting the light beam (whose diameter is equal to the effectivediameter on the light source side of the objective lens) incident on theobjective lens 5, that is, by the sum (d+2×r1) of the thickness d andthe effective diameter (2×r1) on the light source side. Whencompatibility is desired for optical disks of different standards (e.g.,the compatibility between CD/DVD), the working distance WD, that is, theclearance between the objective lens 5 and the optical disk surface,need be increased. However, when WD is increased, the effective radiusr2 on the optical information recording medium side increases. When r2is increased, r1 also increases, thereby inviting an increase in theheight of the optical pickup device.

Thus, with adopting WD/(d+2×r1) as an index, when an objective lens isformed in such a manner that the numerical value of the index shouldbecome greater, contribution can be made for height reduction in theoptical pickup device. Specifically, as shown in the condition (7), itis preferable that the value WD/(d+2×r1) is equal to or greater than0.12. Moreover, in order to further reduce the height of the opticalpickup device, it is preferable that WD/(d+2×r1) is equal to or greaterthan 0.14. In contrast, when the value WD/(d+2×r1) is smaller than 0.12,merely small contribution can be made for height reduction in theoptical pickup device.

Embodiment 2

FIG. 3 is a diagram showing a schematic configuration of a computersystem according to Embodiment 2 of the present invention.

A computer system 10 includes a main body 11, a liquid crystal display12 serving as an output device, and a keyboard 13 serving as an inputdevice. Further, the main body 11 includes a CPU 11 a and an opticaldisk drive 11 b.

FIG. 4 is a block diagram showing a schematic configuration of theoptical disk drive 11 b shown in FIG. 3.

The optical disk drive 11 b includes an optical pickup device 111according to Embodiment 1, an interface 112, a motor 114, a turntable116, and a clamper 115. In FIG. 4, an optical disk 113 is placed on theturntable 116. The interface 112 of the optical disk drive 11 b isconnected through a signal line 11 c to the CPU 11 a.

The CPU 11 a transmits various control signals through the interface 112to the optical pickup device 111 and the motor 114. In accordance withthe control signal, the motor 114 drives and revolves the optical disk113 fixed on the turntable 116 by the clamper 115. On the other hand, inaccordance with the various control signals from the CPU 11 a, theoptical pickup device 111 performs read, write and deletion of data onthe recording layer of the optical disk 113.

The optical disk drive 11 b constituting the computer system 10according to Embodiment 2 employs an optical pickup device 111 describedin Embodiment 1. Thus, in comparison with a conventional optical diskdrive, the size is reduced in the optical disk drive 11 b in the opticalaxis direction of the objective lens. Accordingly, the computer system10 can be constructed compactly.

Here, Embodiment 2 has been described for an exemplary case of anoptical disk drive included in a computer system. However, the objectivelens and the optical pickup device according to Embodiment 1 areapplicable to an arbitrary information system that stores information,such as an optical disk player, an optical disk recorder, a carnavigation system, an authoring system, a data server, an AV component,and a vehicle.

Numerical Example

Numerical examples in which the objective lens according to Embodiment 1was implemented practically are described below. In the numericalexamples, the units of lengths relevant to the dimensions of the lens inrespective tables are all “mm”. Further, in the numerical examples, theaspheric surface shape is defined by the following formula.

$X = {\frac{C_{j}h^{2}}{1 + \sqrt{1 - {\left( {1 + K_{j}} \right)C_{j}^{2}h^{2}}}} + {\sum{A_{j,n}h^{n}}}}$Here, the meaning of each symbol is as follows.

h is a height from the optical axis,

X is a distance from a point on the aspheric surface whose height fromthe optical axis is h to the tangential plane at the aspheric surfacevertex,

C_(j) is a curvature at the aspheric surface vertex of the j-th surfaceof the objective lens (C_(j)=1/R_(j) when the curvature radius at theaspheric surface vertex of the j-th surface of the objective lens isdenoted by R_(j)),

K_(j) is the conic constant of the j-th surface of the objective lens,and

A_(j,n) is the n-th aspherical coefficient of the j-th surface of theobjective lens.

Table 1 shows the lens data of the objective lens according to NumericalExamples 1 to 4 and the values corresponding to the individualconditions. In Table 1, λ indicates a design wave length, while nindicates the refractive index of the lens material with respect tolight having the design wave length λ.

TABLE 1 (lens data and values corresponding to individual conditions)NUMERICAL NUMERICAL NUMERICAL NUMERICAL EXAMPLE 1 EXAMPLE 2 EXAMPLE 3EXAMPLE 4 NA 0.85 0.85 0.85 0.85 λ (nm) 405 405 405 405 n 1.6234 1.70971.7097 1.6234 f (mm) 1.408 1.765 2.5 1.408 d (mm) 1.54 1.66 2.45 1.45D_(H-S) (mm) 1.854 2.439 3.488 1.854 D_(H-H) (mm) 0.446 0.674 0.9880.447 D_(H-T2) (mm) 0.853 0.929 1.409 0.812 D_(T1-H) (mm) 0.241 0.0270.053 0.191 R1 (mm) 0.976 0.1268 1.803 0.966 r1 (mm) 1.2 1.5 2.125 1.2r2 (mm) 0.904 1.191 1.631 0.914 WD (mm) 0.5 0.7521 1.0367 0.5423 (1)D_(H-S)/D_(H-H) 4.157 3.619 3.53 4.148 (2) D_(H-T2)/D_(T1-H) 3.53934.407 26.585 4.251 (3) d/f 1.094 0.941 0.98 1.03 (4) R1/r1 0.814 0.8450.848 0.805 (5) r2/r1 0.753 0.794 0.768 0.761 (6) α (°) 62.3 61.3 60.963.5 (7) WD/(d + 2 × r1) 0.127 0.161 0.155 0.141

Tables 2 to 5 show the aspherical data of the objective lens accordingto Numerical Examples 1 to 4.

TABLE 2 (Numerical Example 1) OPTICAL INFORMATION LIGHT SOURCE SIDERECORDING MEDIUM SIDE SURFACE (1ST SURFACE) SURFACE (2ND SURFACE) R0.976456 −3.453415 K −0.9759662 −142.4346 A4 0.072677311 0.23396804 A60.019913925 −0.60371079 A8 −0.003280565 1.3104318 A10 0.037635639−2.3369295 A12 −0.07947901 2.6070762 A14 0.11397914 −1.5876507 A16−0.10277546 0.40283933 A18 0.05421167 — A20 −0.013127114 —

TABLE 3 (Numerical Example 2) OPTICAL INFORMATION LIGHT SOURCE SIDERECORDING MEDIUM SIDE SURFACE (1ST SURFACE) SURFACE (2ND SURFACE) R1.267711 −44.8376 K −0.9781575 −26795.78 A4 0.035822796 0.1082869 A60.0055122 −0.18078275 A8 −8.89971E−05 0.27333919 A10 0.004397701−0.3028286 A12 −0.006375722 0.19845835 A14 0.005824615 −0.069928387 A16−0.003192219 0.010235832 A18 0.001020656 — A20 −0.00015437 —

TABLE 4 (Numerical Example 3) OPTICAL INFORMATION LIGHT SOURCE SIDERECORDING MEDIUM SIDE SURFACE (1ST SURFACE) SURFACE (2ND SURFACE) R1.802685 −47.52278 K −0.9563194 −14670.65 A4 0.011955319 0.04105487 A60.00084973 −0.035017292 A8 0.00013211 0.02486749 A10 6.95402E−05−0.013311584 A12 −8.33011E−05 0.004353366 A14 4.90683E−05 −0.000781434A16 −1.57705E−05 5.90785E−05 A18 2.75935E−06 — A20 −2.13751E−07 —

TABLE 5 (Numerical Example 4) OPTICAL INFORMATION LIGHT SOURCE SIDERECORDING MEDIUM SIDE SURFACE (1ST SURFACE) SURFACE (2ND SURFACE) R0.966362 −4.095634 K −0.97261 −155.7852 A4 0.074242747 0.23736105 A60.023283506 −0.63818663 A8 −0.009256113 1.4606853 A10 0.051302282−2.4381089 A12 −0.09490531 2.4382605 A14 0.12268312 −1.1315584 A16−0.10387775 0.29465413 A18 0.05455745 — A20 −0.013600226 —

FIGS. 5, 7, 9 and 11 are longitudinal aberration diagrams of theobjective lenses according to Numerical Examples 1, 2, 3 and 4,respectively. Further, FIGS. 6, 8, 10 and 12 are lateral aberrationdiagrams of the objective lenses according to Numerical Examples 1, 2, 3and 4, respectively.

As described above, each objective lens according to Numerical Examples1 to 4 is designed such as to satisfy the above-mentioned conditionformulas. Thus, according to the present invention, even in the casewhere NA is remarkably high, thickness and weight of the objective lenscan be reduced and mass productivity can be improved. Moreover,according to the present invention, the occurrence of off-axialaberration and coma aberration resulting from decentering can besuppressed.

The present invention has been described above in detail. However, thedescription given above is merely illustrative examples of the presentinvention from all points of view, and does not limit the scope of thepresent invention. It cannot be overemphasized that various improvementsand modifications can be made without deviating from the scope of thepresent invention.

1. An objective lens for converging light onto a recording surface of anoptical information recording medium in order to perform at least one ofrecording, reproduction and deletion of information on the opticalinformation recording medium, wherein the objective lens is a bi-convexsingle lens having at least one aspheric surface, and satisfies thefollowing conditions:3.5<D _(H-S) /D _(H-H′)<4.3  (1)3.5<D _(H′-T2) /D _(T1-H)<50  (2)0.9<d/f<1.1  (3) (here, a numerical aperture NA on the opticalinformation recording medium side of the objective lens satisfiesNA≧0.85) where, D_(H-S) is a distance [mm] on the optical axis from afront principal point H of the objective lens to a focal point S on anoptical information recording medium side of the objective lens,D_(H-H′) is a distance [mm] on the optical axis from the front principalpoint H of the objective lens to a rear principal point H′ of theobjective lens, D_(H′-T2) is a distance [mm] on the optical axis fromthe rear principal point H′ of the objective lens to an intersectingpoint T2 of the optical axis and the optical information recordingmedium side surface of the objective lens, D_(T1-H) is a distance [mm]on the optical axis from an intersecting point T1 of the optical axisand a light source side surface of the objective lens to the frontprincipal point H of the objective lens, d is a thickness [mm] on theoptical axis of the objective lens, and f is a focal length [mm] of theobjective lens.
 2. The objective lens as claimed in claim 1, satisfyingthe following condition:0.75<r2/r1<0.8  (5) where, r1 is an effective radius [mm] on a lightsource side of the objective lens, and r2 is an effective radius [mm] onan optical information recording medium side of the objective lens. 3.The objective lens as claimed in claim 2, wherein the objective lens isformed from a plastic material.
 4. The objective lens as claimed inclaim 1, wherein the objective lens is formed from a glass material. 5.The objective lens as claimed in claim 1, satisfying the followingcondition:60°<α<65°  (6) where, α is a maximum angle formed between an incidentlight and a normal line that passes a point on the light source sidesurface of the objective lens, a height from the optical axis to thepoint on the light source surface side being equal to or less than theeffective radius on the light source side.
 6. An optical pickup devicefor performing at least one of recording, reproduction and deletion ofinformation on an optical information recording medium, comprising: alight source; and a converging optical system which includes anobjective lens for converging a light beam emitted from the light sourceonto a recording surface of the optical information recording medium,wherein the objective lens is a bi-convex single lens having at leastone aspheric surface, and satisfies the following conditions:3.5<D _(H-S) /D _(H-H′)<4.3  (1)3.5<D _(H′-T2) /D _(T1-H)<50  (2)0.9<d/f<1.1  (3) (here, a numerical aperture NA on the opticalinformation recording medium side of the objective lens satisfiesNA≧0.85) where, D_(H-S) is a distance [mm] on the optical axis from afront principal point H of the objective lens to a focal point S on theoptical information recording medium side of the objective lens,D_(H-H′) is a distance [mm] on the optical axis from the front principalpoint H of the objective lens to a rear principal point H′ of theobjective lens, D_(H′-T2) is a distance [mm] on the optical axis fromthe rear principal point H′ of the objective lens to an intersectingpoint T2 of the optical axis and the optical information recordingmedium side surface of the objective lens, D_(T1-H) is a distance [mm]on the optical axis from an intersecting point T1 of the optical axisand a light source side surface of the objective lens to the frontprincipal point H of the objective lens, d is a thickness [mm] on theoptical axis of the objective lens, and f is a focal length [mm] of theobjective lens.
 7. The optical pickup device as claimed in claim 6,wherein the objective lens satisfies the following condition:0.75<r2/r1<0.8  (5) where, r1 is an effective radius [mm] on a lightsource side of the objective lens, and r2 is an effective radius [mm] onthe optical information recording medium side of the objective lens. 8.The optical pickup device as claimed in claim 7, wherein the objectivelens is formed from a plastic material.
 9. The optical pickup device asclaimed in claim 6, wherein the objective lens is formed from a glassmaterial.
 10. The optical pickup device as claimed in claim 6, whereinthe objective lens satisfies the following condition:60°<α<65°  (6) where, α is a maximum angle formed between an incidentlight and a normal line that passes a point on the light source sidesurface of the objective lens, a height from the optical axis to thepoint on the light source surface side being equal to or less than theeffective radius on the light source side.
 11. The optical pickup deviceas claimed in claim 6, satisfying the following condition:WD/(d+2×r1)≧0.12  (7) where, WD is a distance [mm] from the opticalinformation recording medium side surface of the objective lens to asurface of the optical information recording medium, d is a thickness[mm] on the optical axis of the objective lens, and r1 is an effectiveradius [mm] on a light source side of the objective lens.